The present invention relates in general to analog trim circuits and, more particularly, to a technique of previewing the analog trim results before blowing a fuse to lock the trim in place.
In manufacturing analog integrated circuits, the basic building blocks are usually not accurately controlled by the manufacturing process as may be desired. For example, capacitors and resistors may have the wrong value, and MOS transistors may have the wrong gain setting. There are too many variables in the manufacturing process to yield absolute predictable results. Yet historically analog circuits often require very accurate voltage references, frequency references, and accurately ratioed elements.
To compensate for the process variability, many electronic circuits use analog trimming during test to set resistor values as necessary for proper operation of the circuit. A typical trimming technique utilizes a resistor ladder comprising a series of serially coupled resistors each in parallel with either a fuse or anti-fuse. A fuse is a device that is substantially an electrical short until it is blown open. An anti-fuse is an electrical open until blown when it becomes substantially an electrical short.
The fuse-blowing approach may take several forms, each with its own shortcomings. Laser fuses may be used directly across each resistor element in the ladder to enable and disable conduction through the resistor. During test, certain resistors are selected to open the shunt element thereby adding resistance to the serial path. The resistor ladder should be adjustable at wafer test over a range from say 10 to 2,560 ohms in 10 ohm increments.
The analog trimming may be performed iteratively, i.e. test, trim, test, trim, to measure the effect of the course trim and determine the necessary fine trimming. For iterative trimming, a laser trim system is typically installed on the wafer tester to alternately test and trim. However, one laser system per tester is very expensive. The laser is often in an idle state waiting for the tester. Moreover, if either the test system or laser breaks down both are inoperative.
An alternate approach is to use a zener anti-fuse across the resistor ladder. Such an element can be cheaply trimmed on the tester so that iterative testing can be done in one pass on the tester. Zener anti-fuses require large currents to program. Therefore, each anti-fuse requires its own external pad and probe card needle. This restricts the programming bit count to say 5-10 bits before the die area for test pads and complexity of the probe card requirements become prohibitive.
In general, iterative testing is a slow and expensive process. Consequently, many trimming techniques utilize only a single pass to evaluate which resistors in the serial string should be included to achieve the desired analog circuit operation. Thus, as result of a test measurement, the user blows the shunt fuse elements whereby the circuit is expected to operate as planned. The process of blowing the fuses typically involves laser trimming off-line from the test set to cut the poly material and open the shunt element. The circuit may be returned to the test set to verify proper trimming. If the subsequent testing should fail, the part is typically discarded since it is difficult to patch the shunt fuse elements.
Hence, a need exists for an iterative trimming to evaluate the results of test before permanently setting the trim.